Field of the Invention
The present invention relates to an image processor, an inkjet printing apparatus, a data generating apparatus, and an image processing method. In particular, the present invention relates to an image processing for reducing the color unevenness caused by the variation in the ejecting characteristic among a pluralitie of nozzles for ejecting ink.
Description of the Related Art
Printing heads used in ink jet-type printing apparatuses may vary, due to errors during the manufacture thereof for example, among a plurality of nozzles with regard to the ejecting characteristic (e.g., an ejection volume or an ejecting direction). Such variation tends to cause the resultant printed image to have a density unevenness.
Conventionally, as a processing for reducing such a density unevenness, the use of a head shading technique as disclosed in Japanese patent Laid-Open No. H10-013674 (1998) has been known. The head shading corrects image data depending on the information regarding the ejecting characteristics of the individual nozzles. By this correction, the number of ink dots that are finally printed can be increased or reduced depending on each nozzle to thereby provide, in the resultant printed image, a substantially-even density among the nozzles.
However, even when the head shading technique as described above is used to reproduce a multi dimensional color by overlapping two or more types of inks, a so-called color deviation may be caused where a region printed by a nozzle having an ejection volume different from a standard volume has a different color from an originally-intended color to be printed.
For example, in order to print a blue image by using cyan ink of a standard ejection volume and magenta ink of an ejection volume higher than a standard volume, magenta ink having a higher ejection volume than a standard ejection volume is printed on a printing medium in the form of dots larger than those of cyan. When the printing head as described above is subjected to the correction by the head shading (HS processing), magenta is printed in the form of dots of a smaller number than a standard number (i.e., dots of a smaller number than the number of cyan dots). As a result, the resultant blue image region is formed to include both of not-overlapped cyan dots having a standard size and overlapped dots in which magenta dots larger than cyan dots includes therein cyan dots. Thus, the region as described above has a color different from that of a blue image formed by cyan dots and magenta dots of a standard size and a standard number. The reason is that the former image and the latter image are different in the ratio of self-colored cyan to the printing medium, the ratio of self-colored magenta to the printing medium, and the ratio of blue by overlapped cyan and magenta to the printing medium. The fluctuation of the ratio among areas occupied by the respective colors as described above is caused not only by the variation of the ejection volume but also by the variation of the ejecting direction. Specifically, even when the conventional head shading was used and the density unevenness of the cyan image or the magenta image was solved, the resultant blue image represented by overlapping the cyan image and the magenta image could not avoid the color difference due to the variation of the ejecting characteristic. Furthermore, since the degree of a color difference is different among regions printed by nozzles having different ejecting characteristics, the respective regions that should have the same color show different colors to be recognized, thus resulting in the recognition of the color banding.